somehow something got lost when you went back to windows vista.
For sure, you don't have to pay anything to get that back
If you really want to use windows media player and dvd playback is the only thing you want to add, then you can install something like this:
Free-Codecs.com :: DScaler MPEG Filters 0.0.8 : Free MPEG2 video decoder codec. Install Dscaler MPEG Filters and play MPEG2 files
Should do the trick.
Other than that, I could recommend a codec pack such as cccp or vista codecs package if you want to install codecs for more formats as well, or some people would recomment a player with built in codecs such as vlc, gomplayer or kmplayer.